This invention generally relates to vehicle suspension systems and more particularly to an improved rear torsion spring suspension system for a snowmobile.
The principal function of any vehicle suspension system is to produce riding characteristics acceptable to vehicle occupants throughout the entire speed range of the vehicle and on the various types of terrain over which the vehicle will operate. A snowmobile suspension must be able to accommodate snow surfaces which vary from xe2x80x9chard packxe2x80x9d to powder, terrain which varies from frozen lake surfaces to unimproved backwoods trails, and speed ranges which well exceed 100 mph. The vehicles normally have a suspension associated with each of two forward skis and a rear suspension disposed between the drive track and the body. This disclosure focuses on the rear suspension system.
As with most suspension systems, the heart of a snowmobile rear suspension is a tuned combined of springs and shock absorbers. One of the problems that engineers in the field must deal with in designing such suspensions is the relatively limited vertical distance available for suspension travel. That factor, coupled with the geometry of the track drive system, has made the torsion spring, as opposed to the coil or leaf spring, a popular choice as the primary weight bearing spring. Ideally the springs and their associated shock absorbers should be configured to provide a relatively soft ride when the snowmobile is traveling over a series of relatively small, closely spaced bumps but capable of preventing the suspension from being fully collapsed, or xe2x80x9cbottoming outxe2x80x9d, when the vehicle is traveling over large bumps at higher speeds. Ordinarily such performance would require the use of springs having significantly non-linear spring constants. Particularly, the spring constant should remain fixed during the low and intermediate portions of the suspension travel but then should rise rapidly as the suspension approaches maximum deflection. While torsion springs are advantageous in many respects, they tend to have relatively fixed spring constants over the normal operating range.
Accordingly, it is a principal object of this invention to provide for a torsion spring type rear suspension for a snowmobile in which the spring force increases in a non-linear manner as the suspension moves from the extended to collapsed position.
It is a further object of this invention to provide for such a suspension in which the spring rate characteristics are adjustable to accommodate various types of terrain, speed ranges, and operator weight.
It is yet another object of this invention to provide for a novel cam block which, together with other suspension components, will accomplish the above desired objectives and which can be easily retrofitted on existing torsion spring type suspensions.
This invention can be broadly summarized as providing for an improved suspension for a snowmobile of the type utilizing one or more torsion springs as the principal load carrying spring members. The suspension includes a suspension member movable with respect to the body and a torsion spring for biasing the suspension toward its extended position. The spring includes a coil portion and a leg extending from the coil portion. Also included is means for engaging the spring leg at a variable point of contact. As the suspension moves from the extended position toward the collapsed position, that point of contact is displaced with respect to the engaging means and also with respect to the spring leg. Particularly, it is displaced along the spring leg toward the coil portion of the spring as the suspension is collapsed.
According to a more detailed aspect of the invention, the means for engaging includes a cam having a contoured cam surface which is engaged by the spring leg. As the suspension is collapsed, the point of contact of the spring leg moves along the cam and also along the leg toward the coil portion of the spring. The effect of that movement is to reduce the effective length of the leg, thereby producing a non-linear change in spring force as a function of angular rotation of the leg.
According to a yet more detailed aspect of the invention, the suspension includes means for adjusting the cam block with respect to the suspension rails to permit adjustment of the spring force function to compensate for varying terrain, vehicle speed, and operator weight.